1. Field of the Invention
The invention relates generally to systems for multiple access and more particularly to a multiple access system for a network using multicarrier modulation.
2. Description of the Prior Art
One of the key design functions in a data network is the mechanism by which multiple users share the resources of the network. The sharing of the network resources by the users is termed “multiple access”. For a network handling primarily constant bit rate sources, such as voice, a base station (for wireless) or central office (for wireline) can simply allocate a fixed bandwidth periodically to each active subscriber station (SS), based upon a predetermined data rate. For a bursty network, however, such a predetermined allocation results in extremely inefficient usage of the total network bandwidth since all SSs are not active all the time. In a data network, therefore, a method of on-request allocation of resources is generally preferred.
Several kinds of on-request allocation methods have been proposed and studied in the prior art. The simplest method is polling where the central office or base station (BS) polls all SSs at regular intervals to check whether they require uplink (SS to BS) network access for bandwidth, maintenance, or other needs. Polling works well when there are relatively few SSs in the network. However, polling is inefficient when the number of SSs is large because the overhead from polling uses a large percentage of the total network bandwidth. An alternative approach is a purely random access method, for example ALOHA, slotted ALOHA, carrier sense multiple access collision avoidance (CSMA/CA) or carrier sense multiple access collision detection (CSMA/CD). However, such random access methods are not able to guarantee quality of service (QOS) requirements, such as bounded delay, under heavy loading. Moreover, the efficiency of the random access systems is limited due to collisions between SS transmissions.
An important hybrid class of multiple access methods developed recently is that of “contention-based” access. In these methods an SS requests uplink access by transmitting a contention packet in a reserved part of each uplink frame. If the contention packet is received successfully, the office or base responds by making a suitable allocation to the SS. These methods are able to handle bursty data traffic with low overhead and guarantee low delays required to support delay-sensitive traffic. Contention-based methods have therefore become quite popular in present data networks. For example, the data over cable service interface specification (DOCSIS) for a cable modem network uses a contention window based on slotted ALOHA, while the actual uplink allocations are contention-free and scheduled by the office or base.
Meanwhile, multicarrier modulation, in particular orthogonal frequency division multiplexing (OFDM), has been gaining prominence as a physical layer technology for high-speed data networks operating in harsh dispersive channels. While the slotted ALOHA based contention method described above offers reasonable performance with single carrier modulation, the efficiency of this scheme is dramatically reduced when used with multicarrier modulation. This is because, unlike DOCSIS, where contention slots are all of relatively short duration (minislots), an OFDM symbol is, by definition much longer than a single carrier symbol. Furthermore, decoding a contention packet requires the office or base to have an estimate of the channel to the SS, which would require an additional preamble to be transmitted before each contention packet, thereby further decreasing the efficiency of the request process. Both of these factors put together drastically reduce the efficiency of the ALOHA based schemes for OFDM systems.
On the other hand, multicarrier transmission affords some unique opportunities for designing an optimum multiple access mechanism. Specifically, in OFDM/OFDMA besides the time dimension, the frequency dimension is also available to design a request access scheme. This observation has been used in the prior art to design signal formats in which the OFDM modulation is combined with code division multiple access (CDMA) codes for data transmission. It might be envisioned that some of these proposed schemes could also be tailored for request access as well. For example, one could design a system in which more than one contending user is accommodated per OFDM/OFDMA symbol. Unfortunately, such a scheme would suffer from a severe loss of orthogonality over a frequency-selective channel and thus, would not be robust under the constraints imposed by dispersive channels. Alternatively, multiuser detection would be required for optimal performance leading to enormous complexity as well as training requirements. Thus, there continues to be a need for improved techniques of requesting service in a multiple access multicarrier modulation data network.